Microstructure and Allotropic Transformation of Zirconium

Zirconium has a hexagonal close-packed (hcp) crystal structure at room temperature, and it undergoes allotropic transformation to a body-centered cubic (bcc) structure at about 870°C (1600°F). The two microstructure characteristics result in strong anisotropy of zirconium and its alloys. Generally, the low-temperature hcp crystal structure is called alpha phase while the high-temperature bcc crystal structure is called beta phase. The lattice constants for zirconium are: a = b = 323.2 pm, c = 514.7 pm; α = β = 90°, γ = 120°.

Temperature

Phase Present at Oxygen Content of

°C

°F

1640 ppm

1370 ppm

970 ppm

955

1750

β

β

β

930

1710

α+β

β

β

925

1700

α+β

β

β

920

1690

α+β

β

β

915

1680

α

α+β

β

910

1670

α

α+β

β

905

1660

α

α+β

β

895

1640

α

α+β

α+β

890

1630

α

α

α+β

885

1625

α

α

α+β

865

1590

α

α

α

855

1575

α

α

α

*The temperature at which the metal transforms from one phase to another is called allotropic transformation temperature. Small amounts of impurities, particularly oxygen, strong affect the transformation temperatures of zirconium. The table has listed variation in allotropic transformation temperature with oxygen content for unalloyed zirconium.